Refactored DynamicSurveyFormMeta.

Rather than querying the database in DSFM.__call__, the query must
happen in a higher stack frame and the result must be passed to
DSFM.__call__.  This way, scenarios like 'if this survey exists, create
a DSF instance' need only query the database once.
Refactored DynamicSurveyFormMeta again.

Traded a little simplicity for more robustness. If a field type's
declaration can't be found in the global namespace, we will now attempt
to import it from wtforms.